Keep it dry and keep the air moving. Mold needs moisture and still air to grow. A well-ventilated building on a proper base with good drainage rarely has mold problems.
Don't store wet items inside. If you bring in a wet lawnmower or damp camping gear, let the building air out after. Avoid pushing storage tight against all four walls, leave a small gap for airflow. If you notice any musty smell, a dehumidifier or even just opening the doors on a dry day usually clears it up.



